Banned Ozone-Harming Gas Creeps Back, Suggesting a Mystery Source

Government scientists have detected an increase in emissions of an outlawed industrial gas that destroys ozone, potentially slowing progress in restoring the atmosphere’s protective ozone layer.

The scientists say that the increase is likely a result of new, unreported production of the gas, known as CFC-11, probably in East Asia. Global production of CFC-11, which has been used as a refrigerant and in insulating foams, has been banned since 2010 under an environmental pact, the Montreal Protocol.

The protocol was adopted in the late 1980s in response to studies that showed CFC-11 and similar gases, collectively known as chlorofluorocarbons, depleted atmospheric ozone. A layer of ozone in the stratosphere helps filter ultraviolet radiation from the sun that can cause skin cancer.

The Montreal Protocol, often hailed as the most successful international environmental agreement ever enacted, has led to declines in chlorofluorocarbons and an increase in stratospheric ozone. A full recovery of the ozone layer was expected by midcentury.

But if the emissions of CFC-11 continue, recovery could be delayed by about a decade, said Stephen A. Montzka, the lead author of a report detailing the findings published Wednesday in the journal Nature.

“We’re raising the flag to say, look, this is not what we hope happens for the ozone layer,” said Dr. Montzka, a research chemist at the Earth System Research Laboratory in Boulder, Colo., a part of the National Oceanic and Atmospheric Administration.

David Doniger, director of the climate and clean energy program of the Natural Resources Defense Council, an environmental group in Washington, said the new emissions were “bad for the ozone layer and bad for climate change.”

“It’s worrisome that someone’s cheating,” he said.

But Mr. Doniger noted that the Montreal Protocol, which has been signed by nearly 200 countries, has a strong track record of compliance, with countries often reporting their own violations. “There’s a reasonable chance we’ll figure out what’s happening here,” he said.

Keith Weller, a spokesman for the United Nations Environment Program, which helps implement the protocol, said the findings would be presented to the parties to the agreement for review. “It is critical that we take stock of this science, identify the causes of these emissions and take necessary action,” he said.

Although CFC-11 and similar gases have been banned for years, they still leak in small amounts into the atmosphere, largely as buildings and equipment containing insulating foams are demolished or destroyed. CFC-11 has a lifetime of about 50 years, so the limited emissions coupled with natural breakdown of the gas should have caused its concentration in the atmosphere to decline at a more rapid rate every year.

Beginning in 2013, however, analysis of air samples from a dozen NOAA monitoring sites around the world showed that while the CFC-11 concentration was still declining, it was doing so at a slower rate. That, Dr. Montzka said, suggested there were new sources of the gas.

“I saw this and said, ‘I just can’t believe emissions have increased,’ ” he said. So Dr. Montzka spent several years analyzing other potential explanations for the results. One possibility was that large numbers of older buildings containing foam insulation were being demolished; another was that the atmospheric processes that lead to the breakdown of CFC-11 had somehow changed. He found no evidence to support either explanation.

Also in 2013, samples taken in Hawaii showed a sudden increase in CFC-11, suggesting the gas was being produced in East Asia and being blown across the Pacific Ocean.

“Let’s be clear that the CFC-11 concentration is still going down,” Dr. Montzka said. If the new emissions go away soon, he said, “it won’t have much of an impact on the timetable for ozone recovery.”

“I’m hoping that once this flag is raised and awareness increases there will be significant efforts to identify the source,” he said.